This program is part of Netpbm(1).
pbmtox10bm was replaced in Netpbm 10.37 (December 2006) by pbmtoxbm(1).
pbmtoxbm with the -x10 option is backward compatible with pbmtox10bm. pbmtoxbm also can generate X11 bitmaps.
You should not make any new use of pbmtox10bm and if you modify an existing use, you should upgrade to pbmtoxbm. But note that if you write a program that might have to be used with old Netpbm, pbmtox10bm is the only way to do that.